Battery health not improving after replacement?

Hi!

Recently I decided to replace the battery on my S20 5G, I did it using the guide here and all was good, or so I thought.

The health of the old battery according to AccuBattery was around 67%, after installing the new one I reset the cycle counter using the Samsung repair assistant app and let it cycle a couple of times from 0% to 100% to calibrate, also according to the guide here on the site.

Now, my "new", supposedly OEM, battery only showed 85% health after 2 weeks of usage. I discussed with the store I bought it from and they sent me another "new" battery, but after again resetting the cycle counter etc., and 2 weeks later even the second "new" battery is showing 81% health...

Am I doing something wrong here? Or are these batteries bad/used/fakes? I'm lost tbh...

Thanks in advance to anyone who can enlighten me!